The synthesis of bridged bis(indenyl) metallocenes can be challenging. Deprotonation of the pro-ligand followed by introduction of a metal halide, typically, a group 4 metal tetrachloride, generally forms two isomers of the product, racemic (rac) and meso. The meso form is often found to be undesirable because its polymerization activity relative to the rac isomer can be greatly reduced. Further, rac-dimethylsilyl-bridged bis(indenyl) metallocenes are of significant interest due to their ability to catalyze the formation of isotactic polypropylene. Thus, repeated crystallizations may be required to isolate the pure rac product, leading to lengthy purification processes and diminished yields. Directing metallation reagents are available, but can be difficult or expensive to scale, or can be found to be ineffective. Thus, the need for scalable, effective rac-directing metallation reagents still exists.
Nifant'ev et al. have used tert-butylamide as a directing agent for substituted bis indenyl and indacenyl compounds, however the effect with such was inconsistent, especially for 2-methyl, 4 phenyl substituted indenes, see Nifant'ev, I. E. et al., Organometallics, 2012, 31, 4340).
Other references of interest include 1) Damrau, H.-R. H. et al., Organometalleas, 2001, 20, 5258; 2) LoCoco, M. D. et al., J. Am. Chem. Soc., 2004, 126, 15231, and 3) Njua, E. Y. et al., Inorg. Chem., 2010, 49, 2163.